The mouse macrophage cell line P388D1, in conjunction with the potent inflammatory and tumor promoting agent, phorbol myristic acetate (PMA) have been used as a model system for 1) production and characterization of Lymphocyte Activating Factor (LAF) and 2) the analysis of the factors which regulate LAF production and secretion. PMA is a potent stimulant of LAF production by P388D1 cells. The ability of several PMA analogs to enhance LAF production is consistent with their inflammatory and tumor promoting activities. The LAF produced in respnse to PMA is a 15,000 m.w. protein which is quite resistant to a variety of degradative and denaturing agents. LAF may exist within macrophages as a relatively inactive high molecular weight precursor which is converted to the 15,000 m.w. form upon activation. LAF activity in P388D1 cells has been observed with m.w. of 13,000, 26,000, 39,000, 50,000 and 220,000 daltons.